User equipment, a network node and methods therein for performing and enabling device-to-device (d2d) communication in a radio communications network

ABSTRACT

A method performed by a first user equipment for performing Device-to-Device, D2D, communications with a second user equipment is provided. The first user equipment determines that a valid timing reference is not present in the first user equipment. Also, the first user equipment determines whether a preconfigured timing advance can be used in the D2D communication. Then, when determined that the preconfigured timing advance can be used, the first user equipment transmits a D2D signal to the second user equipment with a timing using the preconfigured timing advance to perform D2D communication.
         A user equipment, a network node and method therein for enabling D2D communications between user equipments in a radio communications network are also provided.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/358,849, filed May 16, 2014, which is a 35 U.S.C. §371 national phasefiling of International Application No. PCT/SE2014/050440, filed Apr.10, 2014, which claims the benefit of U.S. Provisional Application Ser.No. 61/810,304, filed Apr. 10, 2013, the disclosures of which areincorporated herein by reference in their entireties.

TECHNICAL FIELD

Embodiments herein relate to Device-to-Device (D2D) communication in aradio communications network. In particular, embodiments herein relateto a user equipment and a method therein for performing D2Dcommunication with another user equipment in a radio communicationsnetwork. Embodiments herein further relate to a network node and amethod therein for enabling D2D communication between user equipments ina radio communications network.

BACKGROUND

In a typical radio communications network, wireless terminals, alsoknown as mobile stations, terminals and/or user equipments, UEs,communicate via a Radio Access Network, RAN, to one or more corenetworks, CNs. The radio access network covers a geographical area whichis divided into cell areas, with each cell area being served by a basestation, e.g. a radio base station, RBS, or network node, which in somenetworks may also be called, for example, a “NodeB” or “eNodeB”. A cellis a geographical area where radio coverage is provided by the radiobase station at a base station site or an antenna site in case theantenna and the radio base station are not collocated. Each cell isidentified by an identity within the local radio area, which isbroadcast in the cell. Another identity identifying the cell uniquely inthe whole mobile network is also broadcasted in the cell. The basestations communicate over the air interface operating on radiofrequencies with the user equipments within range of the base stations.

A Universal Mobile Telecommunications System, UMTS, is a thirdgeneration mobile communication system, which evolved from the secondgeneration, 2G, Global System for Mobile Communications, GSM. The UMTSterrestrial radio access network, UTRAN, is essentially a RAN usingwideband code division multiple access, WCDMA, and/or High Speed PacketAccess, HSPA, for user equipments. In a forum known as the ThirdGeneration Partnership Project, 3GPP, telecommunications supplierspropose and agree upon standards for third generation networks and UTRANspecifically, and investigate enhanced data rate and radio capacity. Insome versions of the RAN as e.g. in UMTS, several base stations may beconnected, e.g., by landlines or microwave, to a controller node, suchas a radio network controller, RNC, or a base station controller, BSC,which supervises and coordinates various activities of the plural basestations connected thereto. The RNCs are typically connected to one ormore core networks.

Specifications for the Evolved Packet System, EPS, have been completedwithin the 3^(rd) Generation Partnership Project, 3GPP, and this workcontinues in the coming 3GPP releases. The EPS comprises the EvolvedUniversal Terrestrial Radio Access Network, E-UTRAN, also known as theLong Term Evolution, LTE, radio access, and the Evolved Packet Core,EPC, also known as System Architecture Evolution, SAE, core network.E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein theradio base station nodes are directly connected to the EPC core networkrather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNCare distributed between the radio base stations nodes, e.g. eNodeBs inLTE, and the core network. As such, the Radio Access Network, RAN, of anEPS has an essentially “flat” architecture comprising radio base stationnodes without reporting to RNCs.

Device discovery is a well-known and widely used component of manyexisting wireless technologies, including ad hoc and cellular networks.Examples comprise Bluetooth and several variants of the IEEE 802.11standards suite, such as, e.g. WiFi Direct. These systems operate in theunlicensed spectrum.

Recently, D2D communications as an underlay to cellular or radiocommunications networks have been proposed as a means to take advantageof the proximity of communicating devices, i.e. UEs, and at the sametime to allow devices to operate in a controlled interferenceenvironment.

Typically, it is suggested that such D2D communication should share thesame spectrum as the cellular or radio communication network. This maybe performed, for example, by reserving some of the cellular or radiouplink, UL, resources for D2D communication purposes. Another solutionmay comprise allocating a dedicated spectrum for D2D communication,which is a less likely alternative as spectrum is a scarce resource;particularly, since dynamic sharing between D2D services andcellular/radio services is more flexible and provides a higher spectrumefficiency.

It becomes clear that for D2D communication to occur, the UE must havethe same understanding of UL subframe timing as the cellular or radiocommunications network. Otherwise, they might overlap in time with thecellular or radio transmissions.

In LTE, like in several other cellular standards, a so-called TimingAdvance, TA, is used to ensure that UL transmissions from different UEsare received at the same time, approximately, at the base station.Thereby, orthogonality between the UEs is maintained.

In essence, the base station is measuring the arrival time oftransmissions from the UEs and, when necessary, transmitting a timingadvance command to the UEs to adjust the transmission timing. At the UE,the timing of downlink, DL, transmissions is known, that is, since theUE is capable of receiving DL transmissions it has established a DLtiming reference. The TA command received by the UE is used to determinethe start of an UL subframe relative to the start of a DL subframe, i.e.the UL timing reference is obtained from the DL timing reference and theTA command. The propagation delay from the base station to UEs far outin a cell is larger, and therefore a larger TA is needed, compared toUEs that are located close to the base station. This is illustrated inFIG. 1.

To maintain orthogonality between the transmissions from different UE,the timing misalignment at the base station should be, significantly,less than the duration of the cyclic prefix. In OFDM-based systems, suchas, e.g. LTE, a cyclic prefix is commonly used to handle time dispersionin the radio channel. Note that the cyclic prefix preferably shouldcover the time dispersion in the channel as well, and that the timingmisalignment allowed must take this into account. If the UE has notreceived a TA command in a configurable time period, the UE declares theUL not to be time synchronized. This may be implemented by starting atimer, such as, a Timing Advance Timer, TAT, at each reception of a TAcommand. When the TAT expires, e.g. reaches zero, the UL is consideredno to be time aligned.

Cellular systems, or radio communications networks, often definemultiple states for the UE which matches different transmissionactivities. For example, in LTE, two states are defined:

RRC_IDLE, where the UE is not connected to a particular cell and no datatransfer may occur in either UL or DL. In this state, the UE is inDiscontinuous Reception (DRX) most of the time except for occasionallymonitoring the paging channel. RRC stands for Radio Resource Control.

RRC_CONNECTED, where the UE is connected to a known cell and may receiveDL transmissions. Although expressed differently in the standardspecifications, this state may be considered to have two sub-states:

-   -   UL_IN_SYNC, where the UE has a valid TA value such that UL        transmissions may be received without collisions between        different UEs; and    -   UL_OUT_OF_SYNC, where the UE does not have a valid TA value and        hence cannot transmit data in the UL. Here, prior to any        transmission, a random access must be performed to synchronize        the uplink.

Furthermore, in LTE, random access is used to achieve UL timesynchronization for a UE which either has not yet acquired, or has lost,its UL synchronization. Once UL synchronization is achieved for a UE,the base station, in this case, a eNodeB, may schedule orthogonal ULtransmission resources for the UE.

Some examples of relevant scenarios in which the Random Access Channel,RACH, is used for the random access are therefore:

-   -   A UE in RRC_CONNECTED state, but not UL synchronized, needing to        send new UL data or control information, such as, for example,        an event-triggered measurement report or a hybrid Automatic        Repeat Request (ARQ) acknowledgement in response to a DL data        transmission;    -   A UE in RRC_CONNECTED state, handing over from its current        serving cell to a target cell;    -   For positioning purposes in RRC_CONNECTED state, when TA is        needed for UE positioning;    -   A transition from RRC_IDLE state to RRC_CONNECTED, such as, for        example, for initial access or tracking area updates;    -   Recovering from a Radio Link Failure, RLF.

For D2D communication, it is necessary to define the transmission andreception timing. In principle, any transmission timing could be used aslong as transmissions do not interfere with cellular communication. Onesolution is to use the same transmission timing at the UE for D2Dtransmissions as for cellular UL transmissions. This ensures that D2Dtransmissions do no collide with UL transmissions from the same UE, andalso avoids a, potentially complicated, additional TA mechanism for theD2D communication.

Please note that the term ‘cellular’ as used herein could be furtherextended to an out-of-network coverage scenario, where the UEs mayestablish a hierarchical structure consisting of UE cluster head, CH,i.e. one UE serving as the CH, and slave UEs controlled by the UEserving as the CH. In this way, the CH in many respects behaves similarto a base station, or eNB in this case, and the concept of ‘cluster’ maybe seen as the ‘cell’ in traditional cellular or radio communicationsnetwork. Hence, in the following, the term ‘cellular’ may be alsoapplied to the hierarchical structure of UEs comprising a CH and slaveUEs.

Even in absence of a cellular connection, i.e. when the UE is in anRRC_IDLE mode, UEs may perform both D2D peer discovery and/or D2Dcommunication data transmissions on a reserved resource pool in order tosave signalling overhead for arbitrary control. Furthermore, despitethat a UE does not need to maintain UL timing after the TAT has expired;UEs may still want to send D2D data. However, these communications maythen add interference within the cell.

SUMMARY

It is an object of embodiments herein to provide a mechanism thatenables D2D communication in an efficient manner.

According to a first aspect of embodiments herein, the object isachieved by a method performed by a first user equipment for performingDevice-to-Device, D2D, communications with a second user equipment isprovided. The user equipment determines that a valid timing reference isnot present in the first user equipment. Also, the user equipmentdetermine whether a preconfigured timing advance can be used in the D2Dcommunication. Then, when determined that the preconfigured timingadvance can be used, the first user equipment transmits a D2D signal tothe second user equipment with a timing using the preconfigured timingadvance to perform D2D communication.

According to a second aspect of embodiments herein, the object isachieved by a first user equipment for performing D2D communicationswith a second user equipment. The first user equipment is configured todetermine that a valid timing reference is not present in the first userequipment. Also, the first user equipment is configured to determinewhether a preconfigured timing advance can be used in the D2Dcommunication. Then, when determined that the preconfigured timingadvance can be used, the first user equipment is configured to transmita D2D signal to the second user equipment with a timing using thepreconfigured timing advance to perform D2D communication.

According to a third aspect of embodiments herein, the object isachieved by a method performed by a network node for enabling D2Dcommunications between user equipments in a radio communicationsnetwork. The network node determines information comprising one or moreof: a list indicating in which cells a preconfigured timing advance isused, an Information Element, IE, indicating whether a preconfiguredtiming advance is used, and a preconfigured timing advance. Also, thenetwork node transmits the determined information in a broadcast messageto at least one of the user equipments.

According to a fourth aspect of embodiments herein, the object isachieved by a network node for enabling D2D communications between userequipments in a radio communications network. The network node isconfigured to determine information comprising one or more of: a listindicating in which cells a preconfigured timing advance is used, an IEindicating whether a preconfigured timing advance is used, and apreconfigured timing advance. Also, the network node is configured totransmit the determined information in a broadcast message to the userequipments.

By having a user equipment determine whether a preconfigured timingadvance may be used for a D2D communication when no valid timing advanceis present, the user equipment is enabled to determine how to establishcorrect uplink timing information for performing the D2D communicationwhich avoids/reduces interference within a cell. Thus, a mechanism thatenables D2D communication in an efficient manner is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the embodiments will become readily apparentto those skilled in the art by the following detailed description ofexemplary embodiments thereof with reference to the accompanyingdrawings, wherein:

FIG. 1 is a schematic block diagram illustrating uplink timing advancein a radio communications network,

FIG. 2 is a schematic block diagram illustrating embodiments of a userequipment

FIG. 3 is a flowchart depicting embodiments of a method in a userequipment,

FIG. 4 is a schematic block diagram illustrating cyclic prefixes,

FIG. 5 is a schematic block diagram depicting embodiments of a userequipment,

FIG. 6 is a flowchart depicting embodiments of a method in a networknode,

FIG. 7 is a schematic block diagram depicting embodiments of a networknode.

DETAILED DESCRIPTION

The figures are schematic and simplified for clarity, and they merelyshow details which are essential to the understanding of the embodimentspresented herein, while other details have been left out. Throughout,the same reference numerals are used for identical or correspondingparts or steps.

FIG. 2 is a schematic overview depicting a radio communications network100. The radio communications network 100 comprises one or more RANs andone or more CNs. The radio communications network 100 may use a numberof different technologies, such as Long Term Evolution (LTE),LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), GlobalSystem for Mobile communications/Enhanced Data rate for GSM Evolution(GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), orUltra Mobile Broadband (UMB), just to mention a few possibleimplementations.

In the radio communications network 100, a first terminal or userequipment, UE 121, also known as a mobile station and/or a wirelessterminal, communicates via a Radio Access Network (RAN) to one or morecore networks (CN). It should be understood by the skilled in the artthat “user equipment” is a non-limiting term which means any wirelessterminal, Machine Type Communication (MTC) device or node e.g. PersonalDigital Assistant (PDA), laptop, mobile, sensor, relay, mobile tabletsor even a small base station communicating within respective cell.

The radio communications network covers a geographical area which isdivided into cell areas, e.g. a cell 115 being served by a radio basestation 110. The radio base station 110 may also be referred to as afirst radio base station or a network node. The radio base station 110may be referred to as e.g. a NodeB, an evolved Node B, eNB, eNode B, abase transceiver station, Access Point Base Station, base stationrouter, or any other network unit capable of communicating with a userequipment within the cell served by the radio base station dependinge.g. on the radio access technology, RAT, and terminology used. Theradio base station 110 may serve one or more cells, such as the cell115.

A cell is a geographical area where radio coverage is provided by theradio base station equipment at a base station site. The cell definitionmay also incorporate frequency bands and radio access technology usedfor transmissions, which means that two different cells may cover thesame geographical area but using different frequency bands. Each cell isidentified by an identity within the local radio area, which isbroadcast in the cell. Another identity identifying the cell 115uniquely in the whole radio communications network 100 is alsobroadcasted in the cell 115. The radio base station 110 communicatesover the air or radio interface operating on radio frequencies with theuser equipment 121 within range of the radio base station 110. The userequipment 121 transmits data over the radio interface to the radio basestation 110 in Uplink, UL, transmissions and the radio base station 110transmits data over an air or radio interface to the user equipment 121in Downlink, DL, transmissions.

In some versions of the radio communications network 100, several basestations are typically connected, e.g. by landlines or microwave, to acontroller node (not shown), such as, e.g. a Radio Network Controller,RNC, or a Base Station Controller, BSC, which supervises and coordinatesvarious activities of the plural base stations connected thereto. TheRNCs are typically connected to one or more core networks.

A second terminal or user equipment, UE 122, is located in proximity ofthe first UE 121. This second UE 122, as well as the first UE121, iscapable of D2D communication. Please note that with the term ‘D2Dcommunication’ or ‘Device-to-device communication’ herein is meant thetransmission of beacon signals, D2D (peer) discovery and D2Dcommunication data transmissions.

It should be noted that some embodiments herein relate tosynchronization in network-assisted D2D communication. Furthermore,although embodiments below are described with reference to the scenarioof FIG. 1, this scenario should not be construed as limiting to theembodiments herein, but merely as an example made for illustrativepurposes.

In accordance with embodiments described herein, the issue of providinga mechanism that enables D2D communication in an efficient manner isaddressed by the first UE 121, being in an idle mode or out-of-syncmode, obtaining UL timing information, such as, TA information. This maybe performed by the first UE 121 either by using a pre-set, stored orpreconfigured TA value or by retrieving the TA information from a radiobase station/network node 110 in a random access process. The first UE121 may then use the UL timing information in a D2D communication withthe second UE 122.

This means that, before the first UE 121, being in an idle mode or an ULout-of-sync mode, is allowed to start D2D communication, including alsoe.g. the transmission of beacon signals, the first UE 121 should either:

-   -   perform a random access to the network node 110 to get e.g. the        TA information; or    -   assume a fixed, preferably small, value for the TA, such as,        e.g. TA=0, when permitted by network signaling and/or UE        measurement.

Hence, embodiments herein describes first and second UEs 121, 122, whenin idle mode, e.g. RRC_IDLE, or out-of-sync mode, e.g. UL_OUT_OF_SYNC,being able to obtain a UL timing information value, e.g. TA value, andtherefore being able to establish a correct UL timing in order toavoid/reduce interference within a cell, e.g. cell 115 in FIG. 1.

Example of embodiments of a method performed by the first UE 121 forperforming D2D communication with the second UE 122 will now bedescribed with reference to the flowchart depicted in FIG. 3. Theembodiments relate to a method for determination of D2D transmissiontiming in the non-presence of valid remote node timing reference.

FIG. 3 is an illustrated example of actions or operations which may betaken or performed by the first UE 121. The method may comprise thefollowing actions.

Action 301

Optionally, in this action, the first UE 121 may determine that a timinginformation is not present, that is, determine that a valid timingreference is not present in the first UE 121. In some embodiments, thefirst UE 121 may not have a valid timing reference because the first UE121 is in an idle mode, e.g. RRC_IDLE, or out-of-sync mode, e.g.UL_OUT_OF_SYNC. In this case e.g. if the first UE 121 does not have avalid uplink timing reference, such as, for example, because it is inidle mode or a too long time has passed since the last received TAcommand, the first UE 121 may acquire uplink synchronization prior todirect D2D communication. Two situations may be thought of when it comesto uplink synchronization, as shown in Action 302.

It should also be noted that the first UE 121 may also, prior to any D2Dtransmission, including transmission of beacon signals, determinewhether it has a valid uplink timing reference and, if the first UE 121has a valid uplink timing reference, then direct D2D transmissions,including beacons and synchronous random access, may take place in theresources assigned.

Action 302

In this action, the first UE 121 obtains timing information. This actioncomprises sub-actions 302 a, 302 b and 302 c.

As indicated in sub-action 302 a, the first UE 121 obtains timinginformation by determining whether a preconfigured timing advance can beused in the D2D communication. When determined that a preconfiguredtiming advance can be used, the first UE 121 proceeds to Action 303 a.

In some embodiments, the first UE 121 may determine whether apreconfigured timing advance is used when the first UE 121 do not have avalid timing reference based on information received in a broadcastmessage from a network node 110. In this case, the information maycomprise one or more of:

-   -   a list indicating in which cells a preconfigured timing advance        is used;    -   a cell size;    -   an Information Element, IE, indicating whether a preconfigured        timing advance is used;    -   a subframe format allocated to a D2D communication indicating        that an extended cyclic prefix is used; and    -   a preconfigured timing advance.

In one example, information about the cell size, or, rather, whether arandom access is required prior to D2D communication, may be included inthe system information, SI, i.e. broadcast message. As part of the cellsearch procedure, the first UE 121 may obtain downlink synchronizationand read the (relevant) system information to determine whether the cellsize is small or large. The requirements may be indicated as cell size,or if required or not, in an information element.

In another example, or as an extension, the first UE 121 could beprovided with a list, reflecting whether random access prior to D2Dcommunication is required in each of the neighboring cells or not toreduce the delay associated with reading system information when doingcell reselection in idle mode. In some embodiments, a list wherepreconfigured TA is valid in a set of cells, including thecamping/serving cell 115, is received from a broadcast message from theremote node, i.e. the network node 110.

With such a list, the first UE 121 could, after acquiring the cellidentity of the neighboring cell, determine whether a random access isneeded or not without reading the system information, SI, in the newcell. Such a list could also, as an alternative or complement toincluding it in the system information, be provided to the first UE 121after, or as part of, the random access procedure when contacting thenetwork node 110 in the first cell. Thus, the list indicating whetherrandom access is required may be transmitted from the network node 110,i.e. radio base station/eNB/CH 110, to the first UE 121 and/or thesecond UE 122.

In a further example, or as an extension, this information on whether apreconfigured or fixed TA may be assumed or not could be subframespecific, and this subframe partition information may be included in thesystem information as well. For example, a set of subframes isconfigured to use extended CP despite not motivated by the propagationconditions, e.g. 16.7 μs, where only 5 μs for time dispersion, to absorbmore timing difference, i.e. 11.7 μs margin provides 1.75 km coverage,especially for beacon transmission. As a further extension, ifinter-cell/cluster sync is available, this fixed TA subframe setconfiguration, e.g. TA=0, may be common/coordinated for neighboringcells, which means the fixed/zero TA beacon Tx/Rx, i.e. transmission andreception, or actual D2D peer discovery, may be even achieved for aninter-cell/cluster scenario.

Furthermore, in some embodiments, the first UE 121 may also determinethat a preconfigured timing advance can be used when received signalstrength measurements in the first UE 121 is above a threshold. In otherwords, UE measurements may be used to differentiate whether zero TA maybe applied or not, for example, if the received 30 Reference SignalReceived Power, RSRP, is larger than a specific threshold. The RSRP herebeing a measure of downlink signal strength used for, e.g. handover inLTE, and the specific threshold being is broadcasted in the systeminformation. In this case, zero TA may be applied directly.

Furthermore, in some embodiments, the usage of preconfigured TA for the35 camping/serving cell 115 may be determined based on signal strengthmeasurements of received signal from the camping/serving cell 115. Inthis case, a preconfigured TA is determined to be used if the signalstrength is above a threshold.

Otherwise, a random access is required to obtain the non-zero TA commandfrom the network node 110. In other words, if not a preconfigured TA isused in the cell, transmit a random access to the remote node, i.e. thenetwork node 110, and receive a TA from the network node 110. Thisenables the first UE 121 to transmit the D2D signal according to thereceived transmit timing, as shown in Action 303.

As indicated in sub-action 302 b, when determined that a preconfiguredtiming advance cannot be used, the first UE 121 may, in someembodiments, obtains timing information by transmitting a random accesssignal to a network node 110.

As indicated in sub-action 302 c, the first UE 121 may, in this case,also receive a valid timing advance from the network node 110.

It should also be noted that the network node 110 referred to may alsobe another UE acting as a Cluster Head, CH, to which the first UE 121 isa slave UE. In other words, the preconfigured TA for the camping/servingcell, i.e. cell 115, is received from a broadcast message from thenetwork node 110, such as, the radio base station/eNB/CH 110 or anotherRAN node. The network node 110 may be a eNodeB, eNB or a Cluster Head,CH. Also, in some embodiments, the timing advance, TA, may be a fixedand/or zero value.

In some embodiments, the first UE 121 may determine whether apreconfigured TA is used in the cell 115. The determination of whether apreconfigured TA may be used 25 may be performed based on a sub frameformat allocated to a D2D communication. In some embodiments, the firstUE 121 may determine that a preconfigured TA may be used if a longcyclic prefix, CP, is used in D2D subframes.

Action 303

this action, the user equipment 121 performs D2D communication. Thisaction comprises sub-actions 303 a and 303 b.

As indicated in sub-action 303 a, this is performed by the first UE 121by, when determined that a preconfigured timing advance can be used,i.e. as in Action 302, transmitting a D2D signal to the second UE 122with a timing using the preconfigured timing advance to perform D2Dcommunication.

In other words, if a preconfigured TA is used, the first UE 121transmits the D2D signal with a timing according to the preconfiguredvalue to the second UE 122.

This is because, in case of smaller cell sizes, longer cyclic prefix,CP, margin and less time dispersion in the channel, the TA required maybe absorbed by the CP. This is shown in the example of FIG. 4.

FIG. 4 shows an example of cyclic prefixes absorbing timinguncertainties and the use of a preconfigured timing advance of zero,i.e. TA=0. Here, a TA of zero, or another small predefined value, issufficient in these situations to ensure that the timing misalignment atthe network node 110, e.g. eNB or CH, is significantly smaller than theduration of the cyclic prefix, CP. According to embodiments herein, thismay be exploited to avoid the random access step, irrespective of thestate of the first UE 121, prior to the D2D transmissions.

As indicated in sub-action 303 b, when a valid timing advance has beenreceived from the network node 110, i.e. as in sub-action 302 c, thefirst UE 121 may perform D2D communication by transmitting a D2D signalto the second UE 122 with a timing using the received valid timingadvance.

This means that obtaining uplink timing synchronization by performing arandom access procedure and receiving a TA command from the network node110, e.g. the radio base station/eNB/CH 110, is always a valid option.In some cases, it is also necessary.

Once the UL timing reference is established, direct D2D transmissionsmay take place in the resources assigned. It should be clarified thatthe random access procedure mentioned herein refers to transmission ofan uplink message followed by at least one response from the networknode 110 consisting of at least a TA command. The random access channelfor D2D operation mentioned herein might carry different content, formatand have transmission procedure as compared to the random accesschannels conventionally employed for cellular operations.

It should be noted that the UE 121 may transmit the D2D signal on achannel for D2D operation.

An advantage with the embodiments mentioned herein, is that one mayensure that D2D transmission is synchronized with the network UL timing,and hence unnecessary interference from D2D between two UEs, e.g. firstand second UE 121, 122, on a third device's network communication isavoided.

To perform the method actions in the first UE 121 for performing D2Dcommunications with the second UE 122, the first UE 121 may comprise thefollowing arrangement depicted in FIG. 5. The first and second UE 121,122 are configured to be in a radio communications network 100, e.g. asshown in FIG. 2.

FIG. 5 shows a schematic block diagram of embodiments of the first UE121. In some embodiments, the first UE 121 may comprise an obtainingmodule 501, a determining module 502, and a performing module 507, whichmay also be referred to as circuits. In some embodiments, the first UE121 may comprise a processing circuit 503, which may also be referred toas processing module, processing unit or processor. The processingcircuit 610 may comprise one or more of the obtaining module 501, thedetermining module 502, and the performing module 507, and/or performthe function thereof described below.

The first UE 121 is configured to, or comprises the determining module502 being configured to, determine that a valid timing reference is notpresent in the first user equipment. Also, the first UE 121 isconfigured to, or comprises the obtaining module 501 being configuredto, determine whether a preconfigured timing advance can be used in theD2D communication. Furthermore, the first UE 121 is configured to, orcomprises the performing module 507 being configured to, transmit a D2Dsignal to the second user equipment 122 with a timing using thepreconfigured timing advance to perform D2D communication whendetermined that the preconfigured timing advance can be used.

In some embodiments, the first UE 121 or obtaining circuit 501 may befurther configured to determine whether a preconfigured timing advancecan be used when the first UE 121 do not have a valid timing referencebased on information received in a broadcast message from a network node110. In this case, the information may comprise one or more of: a listindicating in which cells a preconfigured timing advance is used; a cellsize; an Information Element, IE, indicating whether a preconfiguredtiming advance is used; a subframe format allocated to a D2Dcommunication indicating that an extended cyclic prefix is used; and apreconfigured timing advance.

In some embodiments, the first UE 121 or obtaining module 501 may befurther configured to determine that a preconfigured timing advance isused when a received signal strength measurements in the first UE 121 isabove a threshold.

In some embodiments, the first UE 121 or obtaining module 501 may befurther configured to, when determined that a preconfigured timingadvance cannot be used, transmit a random access signal to a networknode 110. In this case, the first UE 121 or obtaining module 501 mayalso be further configured receive a valid timing advance from thenetwork node 110. Furthermore, in this case, the first UE 121 orperforming module 507 may be further configured to transmit a D2D signalto the second UE 122 with a timing using the received valid timingadvance.

Also, in some embodiments, the timing advance, TA, may be a fixed and/orzero value.

In some embodiments, the first UE 121 or performing module 507 may befurther configured to perform the transmitting of the D2D signal to thesecond UE 122 with a timing using the preconfigured timing advance inorder to perform D2D communication on a channel for D2D operation.

In some embodiments, the first UE 121 may not have a valid timingreference 15 because the first UE 121 is in an idle mode, e.g. RRC_IDLE,or out-of-sync mode, e.g. UL_OUT_OF_SYNC.

In other words, a wireless device, e.g. the first UE 121, is provided toperform the methods disclosed herein. The first UE 121 is capable of D2Dcommunication with a second wireless device, e.g. the second UE 122. Thefirst UE 121 is configured, when being in a mode when the first UE 121has no valid remote node timing reference, to obtain a network timingreference. In one example, the first UE 121 may comprise an obtainingcircuit 501 configured to obtain uplink timing before communicating withthe second UE 122. The first UE 121 may additionally or alternativelycomprise a determining circuit 502 configured to determine whether apreconfigured TA is used in the cell 115. That being the case, the firstUE 121 may perform D2D transmission with the second UE 122 using thepreconfigured TA in a performing circuit 507.

The first UE 121 further comprises a transmitting module 504 and areceiving module 505, which may also be referred to as circuits. Inother words, the first UE 121 may comprise a transmitting circuit 504configured to transmit data to the second UE 122 and/or the radio basestation/eNB/CH 110. The first UE 121 may further comprise a receivingcircuit 505 configured to receive communication or data from the secondUE 122 and/or the radio base station/eNB/CH 110.

The embodiments for performing D2D communications with another userequipment, i.e. the second UE 122, may be implemented through one ormore processors, such as, e.g. the processing circuitry 503 in the firstUE 121 depicted in FIG. 5, together with computer program code forperforming the functions and actions of the embodiments herein. Theprogram code mentioned above may also be provided as a computer programproduct, for instance in the form of a data carrier carrying computerprogram code or code means for performing the embodiments herein whenbeing loaded into the processing circuitry 503 in the first UE 121. Thecomputer program code may e.g. be provided as pure program code in thefirst UE 121 or on a server and downloaded to the first UE 121. Thecarrier may be one of an electronic signal, optical signal, radiosignal, or computer readable storage medium, such as, e.g. electronicmemories like a RAM, a ROM, a Flash memory, a magnetic tape, a CD-ROM, aDVD, a Blueray disc, etc.

Thus, the first UE 121 may further comprise a memory 506, which may bereferred to or comprise one or more memory modules or units. The memory506 may be arranged to be used to store executable instructions anddata, such as, e.g. Timing Advances, TAs, preconfigured TAs, etc., toperform the methods described herein when being executed in the first UE121. Those skilled in the art will also appreciate that the processingcircuitry 503 and the memory 506 described above may refer to acombination of analog and digital circuits, and/or one or moreprocessors configured with software and/or firmware, e.g. stored in thememory 506, that when executed by the one or more processors such as theprocessing circuitry 503 perform the method as described above. One ormore of these processors, as well as the other digital hardware, may beincluded in a single application-specific integrated circuit (ASIC), orseveral processors and various digital hardware may be distributed amongseveral separate components, whether individually packaged or assembledinto a system-on-a-chip (SoC).

In other words, the first UE 121 may comprise a memory 506 that maycomprise one or more memory units and may be used to store, for example,data such as lists, preconfigured TA or TA, applications to perform themethods herein when being executed on the first UE 121 or similar. Theembodiments herein for enabling D2D communication may be implementedthrough one or more processors, such as a processing circuit 503 in thefirst UE 121, together with computer program code for performing thefunctions and/or method steps of the embodiments herein. The programcode mentioned above may also be provided as a computer program product,for instance in the form of a data carrier carrying computer programcode for performing embodiments herein when being loaded into the firstUE 121. One such carrier may be in the form of a CD ROM disc. It ishowever feasible with other data carriers such as a memory stick. Thecomputer program code may furthermore be provided as pure program codeon a server and downloaded to the first UE 121.

Those skilled in the art will also appreciate that the various“circuits” described may refer to a combination of analog and digitalcircuits, and/or one or more processors configured with software and/orfirmware (e.g., stored in memory) that, when executed by the one or moreprocessors, perform as described above.

Example of embodiments of a method performed by a network node 110 forenabling D2D communications between user equipments, i.e. the first andsecond UE 121, 122, in a radio communications network 100, will now bedescribed with reference to the flowchart depicted in FIG. 6.

FIG. 6 is an illustrated example of actions or operations which may betaken by the network node 110 or a UE acting as a Cluster Head, CH.However, it should also be noted that these actions or operations mayalso be performed by a centralized network node in the radiocommunications network 100, such as, e.g. a core network node, a radionetwork controller, a Radio Resource Management, RRM, server, anOperations Support System, OSS, node or the like. The centralizednetwork node may also be e.g. an eNB controlling distributed RemoteRadio Units, RRUs, via e.g. a Common Public Radio Interface, CPRI, or aneNB controlling radio heads over an active Distributed Antenna System,DAS, network. The method may comprise the following actions.

Action 601

In this action, the network node 110 determines information to bebroadcasted to the first UEs, that is, the network node 110 determinesinformation comprising one or more of: a list indicating in which cellsa preconfigured timing advance is used; an Information Element, IE,indicating whether a preconfigured timing advance is used; and apreconfigured timing advance.

Action 602

In this action, the network node 110 transmits information in abroadcast to the UEs, that is, the network node 110 transmits thedetermined information in a broadcast message to at least one of theuser equipments, i.e. the first and second UE 121, 122.

To perform the method actions in the network node 110 for enabling D2Dcommunications between user equipments, i.e. the first and second UE121, 122, in a radio communications network 100, the network node 110may comprise the following arrangement depicted in FIG. 7. The networknode 110 is configured to be in a wireless communications network, suchas, e.g. the radio communications network 100 in FIG. 1.

FIG. 7 shows a schematic block diagram of embodiments of the networknode 110. In some embodiments, the network node 110 may comprise adetermining module 701, and a transceiving module 702. In someembodiments, the network node 110 may comprise a processing circuit 710,which may also be referred to as processing module, processing unit orprocessor. The processing circuit 710 may comprise one or more of thedetermining module 701 and transceiving module 702, and/or perform thefunction thereof described below. In some embodiments, the network node110 may be a UE acting as a Cluster Head, CH.

The network node 110 is configured to, or comprises a determining module701 being configured to, determine information comprising one or moreof: a list indicating in which cells a preconfigured timing advance isused, an Information Element, IE, indicating whether a preconfiguredtiming advance is used, and a preconfigured timing advance. Also, thenetwork node 110 is configured to, or comprises a transceiving module702 being configured to, transmit the determined information in abroadcast message to the user equipments, i.e. the first and second UE121, 122.

The embodiments for enabling D2D communications between user equipments,i.e. the first and second UE 121, 122, may be implemented through one ormore processors, such as, e.g. the processing circuitry 710 in thenetwork node 110 depicted in FIG. 7, together with computer program codefor performing the functions and actions of the embodiments herein. Theprogram code mentioned above may also be provided as a computer programproduct, for instance in the form of a data carrier carrying computerprogram code or code means for performing the embodiments herein whenbeing loaded into the processing circuitry 710 in the network node 110.The computer program code may e.g. be provided as pure program code inthe network node 110 or on a server and downloaded to the network node110. The carrier may be one of an electronic signal, optical signal,radio signal, or computer readable storage medium, such as, e.g.electronic memories like a RAM, a ROM, a Flash memory, a magnetic tape,a CD-ROM, a DVD, a Blueray disc, etc.

Thus, the network node 110 may further comprise a memory 720, which maybe referred to or comprise one or more memory modules or units. Thememory 720 may be arranged to be used to store executable instructionsand data, such as, e.g. a list indicating in which cells a preconfiguredtiming advance is used, an IE indicating whether a preconfigured timingadvance is used, preconfigured timing advances, TAs, etc., to performthe methods described herein when being executed in the network node110. Those skilled in the art will also appreciate that the processingcircuitry 710 and the memory 720 described above may refer to acombination of analog and digital circuits, and/or one or moreprocessors configured with software and/or firmware, e.g. stored in thememory 720, that when executed by the one or more processors such as theprocessing circuitry 710 perform the method as described above. One ormore of these processors, as well as the other digital hardware, may beincluded in a single application-specific integrated circuit (ASIC), orseveral processors and various digital hardware may be distributed amongseveral separate components, whether individually packaged or assembledinto a system-on-a-chip (SoC).

The terminology used in the detailed description of the particularexemplary embodiments illustrated in the accompanying drawings is notintended to be limiting of the described the methods, the network node110, and user equipment 121, which instead should be construed in viewof the enclosed claims.

As used herein, the term “and/or” comprises any and all combinations ofone or more of the associated listed items.

Further, as used herein, the common abbreviation “e.g.”, which derivesfrom the Latin phrase “exempli gratia,” may be used to introduce orspecify a general example or examples of a previously mentioned item,and is not intended to be limiting of such item. If used herein, thecommon abbreviation “i.e.”, which derives from the Latin phrase “idest,” may be used to specify a particular item from a more generalrecitation. The common 30 abbreviation “etc.”, which derives from theLatin expression “et cetera” meaning “and other things” or “and so on”may have been used herein to indicate that further features, similar tothe ones that have just been enumerated, exist.

As used herein, the singular forms “a”, “an” and “the” are intended tocomprise also the plural forms as well, unless expressly statedotherwise. It will be further understood that the terms “includes,”“comprises,” “including” and/or “comprising,” when used in thisspecification, specify the presence of stated features, actions,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,actions, integers, steps, operations, elements, components, and/orgroups thereof.

Unless otherwise defined, all terms comprising technical and scientificterms used herein have the same meaning as commonly understood by one ofordinary skill in the art to which the described embodiments belongs. Itwill be further understood that terms, such as those defined in commonlyused dictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

The embodiments herein are not limited to the above described preferredembodiments. Various alternatives, modifications and equivalents may beused. Therefore, the above embodiments should not be construed aslimiting.

1. A method performed by a first user equipment for performing aDevice-to-Device (D2D) communication with a second user equipment, themethod comprising: determining that a valid timing reference forcellular network synchronization with a cellular network is not presentin the first user equipment, wherein the first user equipment does nothave a valid timing reference because the first user equipment is in anidle mode; determining that a preconfigured timing advance for cellularnetwork synchronization with the cellular network can be used in the D2Dcommunication; and transmitting a D2D signal to the second userequipment with a timing using the preconfigured timing advance in theD2D communication.
 2. The method according to claim 1, wherein thepreconfigured timing advance is a fixed and/or zero value.
 3. The methodaccording to claim 1, wherein the transmitting is performed on a channelfor D2D operation.
 4. A first user equipment comprising a processingcircuit for performing Device-to-Device (D2D) communications with asecond user equipment, wherein the processing circuit in the first userequipment is configured to determine that a valid timing reference forcellular network synchronization with a cellular network is not presentin the first user equipment where the first user equipment does not havea valid timing reference because the first user equipment is in an idlemode, determine that a preconfigured timing advance for cellular networksynchronization with the cellular network can be used in the D2Dcommunication, and transmit a D2D signal to the second user equipmentwith a timing using the preconfigured timing advance to perform D2Dcommunication.
 5. The first user equipment according to claim 4, whereinthe preconfigured timing advance is a fixed and/or zero value.
 6. Thefirst user equipment according to claim 4, further configured to performthe transmitting of the D2D signal to the second user equipment with atiming using the preconfigured timing advance in order to perform D2Dcommunication on a channel for D2D operation.